Almost all electric and electronic apparatus ranging from motors and electric generators to printed circuit boards comprise conductors to convey electricity and insulating materials.
Recently, these apparatus have been downsized rapidly and required insulating materials of higher characteristics. Further, as the apparatus are downsized and their conductors are packed more densely, they generate more heat. To dissipate this lot of heat has been a great problem to be solved.
Conventionally, thermosetting resin compounds have been used widely as insulating materials for various electric and electronic apparatus, judging from their high insulating characteristics, easy molding capability, and high thermal resistance,
Generally, however, the thermal conductivity of the thermosetting resin is low and not enough to dissipate a lot of heat of highly-densed conductors. Therefore, high-conductivity insulating materials have been much required.
Compounds made of a mixture of a thermosetting resin and highly thermal-conductive filler powder have been well-known as high-conductivity materials. There are various kinds of filler powder under study: powder of metal such as silver or aluminum and powder of inorganic ceramic such as silicon dioxide, aluminum oxide, magnesium oxide, beryllium oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide, aluminum fluoride, or calcium fluoride.
However, filler powder in a thermosetting resin will increase the viscosity of the resin strikingly, make it difficult to manufacture microstructures of resin, and reduce their workability.
Further, the thermal conductivity of the thermosetting resin is extremely low and a little amount of filler powder does not improve the thermal conductivity of the resin so much. To improve the thermal conductivity of the resin, a large quantity of filler material must be added to the resin. However, it is substantially difficult to disperse such a large quantity of filler powder uniformly in the resin.
Furthermore, in most cases, the affinity of filler powders to the thermosetting resin that is one of organic compounds is not so strong. This may easily cause interfacial separation between the filler powder and the resin and consequently reduce the insulation performance of the resin dramatically when it is used for a long time.
From this point of view, it is very important to prepare organic materials of high thermal conductivity.
A preparation of a high thermal-conductivity plastic compound filled with super-highly oriented polymer fibers has been disclosed as a method for preparing organic materials of high thermal conductivity by Japanese Laid-Open Patent Publication No. Sho 61-296068. This method uses a characteristic that super-highly oriented polymer fibers described in POLYMER Vol.19, page 155 (1978) improve the thermal conductivity along their orientation axis.
However, the super-highly oriented polymer fibers are not so thermal-conductive across the orientation axis. So, if dispersed randomly in the organic insulating compounds, such polymer fibers will seldom or never improve the thermal conductivity of the compound.
It is possible to prepare an organic insulating material having an excellent thermal conductivity along the orientation of polymer fibers by orienting polymer fibers in a certain direction in the organic insulating compound, but the thermal conductivity of the insulating material falls in the other directions.
Further, ADVANCED MATERIALS Vol.5, page 107 (1993) and Germany Patent Specification 4226994 disclose anisotropic materials having a high thermal conductivity in the in-plane direction of the film in which molecular chains are disposed, which are prepared by orienting monomer having mesogen groups such as diacrylate and bridging thereof. However, this material has less thermal conductivity in the other directions, particularly, in the in-depth direction of the film.
Generally, as for film materials, most of heat transfers in the in-depth direction of the film material. Therefore, this kind of material is not effective for electric and electronic apparatus.
There has also been an approach to a method for disposing molecular chains in the in-depth direction. Japanese Laid-Open Patent Publication No. Hei 01-149303, Hei 02-5307, Hei 02-28352, and Hei 02-127438 disclose methods for manufacturing organic materials such as polyoxymethylene and polyimide in the presence of an electrostatic potential.
Further, Japanese Laid-Open Patent Publication No. Sho 63-264828 discloses a material having molecular chains oriented in the in-depth direction by laminating molecule-oriented polypropylene or polyethylene sheets with their molecules oriented in the same direction, bonding thereof firmly, and slicing thereof perpendicularly to the orientation of the molecular chains.
Indeed, these methods can prepare materials having high thermal conductivity in the in-depth direction of the film, but their molding becomes very complicated and only a limited number of such materials are available.
Japanese Laid-Open Patent Publication No. Hei 11-323162 discloses an insulating compound having a thermal conductivity of 0.4 W/m·K or more prepared by polymerizing mesogen-structured monomer.
This insulating compound is spatially almost isotopic and high in thermal conductivity, but can increase the in-depth thermal conductivity without any complicating molding method. The thermal conductivity of this compound is about 0.4 to 0.5 W/m·K.
Japanese Laid-Open Patent Publication No. Hei 09-118673 discloses a liquid crystal thermosetting monomer having two mesogen groups and a liquid crystal thermosetting polymer having a smectic structure prepared from this monomer. This liquid crystal thermosetting polymer can form a smectic structure which is oriented highly, but Japanese Laid-Open Patent Publication No. Hei 09-118673 never refers to the thermal conductivity of this polymer.
Judging from the above, it is an object of the present invention to provide a highly thermal-conductive thermoset resin whose thermal conductivity is increased highly while keeping an almost isotopic spatial structure.